1. Field of the Invention
The present invention relates to a current control circuit that controls a current flowing through a primary coil of a transformer, and in particular to a current control circuit that prevents a reverse current induced by a back electromotive force exerted by the primary coil of the transformer.
2. Related Art
CCFLs (Cold Cathode Fluorescent Lamps) are widely utilized for liquid crystal backlights. As CCFLs must be supplied with an alternating current, typically a primary coil of a transformer is supplied with an alternating current to cause the CCFL connected to a secondary coil to emit light. Accordingly, a circuit is required which supplies an alternating current to the primary coil of the transformer.
An example configuration of such a circuit is a push-pull amplifier as shown in FIG. 3. In this circuit, a P channel transistor Q1 is provided between a power source VDD and an output terminal. A diode SBD and an N channel transistor Q2 are arranged between the output terminal and a ground. The transistor Q1 is turned on, while the transistor Q2 is turned off, to allow a current from the power source VDD to flow out from the output terminal. The transistor Q1 is turned off, while the transistor Q2 is turned on, to allow a current to be drawn from the output terminal.
The primary coil of the transformer is connected to the output terminal, and the CCFL is connected to the secondary coil. Thus, by supplying a predetermined alternating current to the primary coil of the transformer, it is possible to allow the CCFL connected to the secondary coil to emit light. A drive circuit for the CCFL is described in Japanese Patent Laid-Open No. 2002-289385.
In such a circuit as described above, if the transistor Q2 is turned on or off, a relatively high reverse voltage is applied to the diode SBD. On the other hand, when the transformer Q2 is turned on, a relatively large current flows through the circuit. In, for example, a backlight for a liquid crystal display in a portable apparatus or the like, a peak current is often at least 10 A. Thus, a Schottky barrier diode (SBD) is normally employed as the diode SBD. However, as heat or resistance from the diode SBD is disadvantageous, the diode SBD must have a large size. For example, the diode SBD must be of, for example, an SMP (Surface Mount Package) class. This is disadvantageous in terms of space and also disadvantageously increases costs.